The present invention relates to a powder coating composition. Further in detail, the powder coating composition of the present invention is suitable for coating of a precoat metal (hereinafter referred to as xe2x80x9cPCMxe2x80x9d).
Hitherto, there have been many proposals as to a powder coating composition. In JP-A-1-103670, for example, there is described a thermosetting powder coating composition comprising a fluorine-containing copolymer containing chlorotrifluoloethylene as the main component and a curing agent.
A coating film obtained from the powder coating composition described in the above-mentioned publication has high gloss and good weather resistance. There is a problem, however, that the coating film is inferior in bending resistance because the fluorine-containing copolymer is crosslinked by the curing agent, and thus the coating film is difficult to be employed for an application such as PCM which need to be subjected to post processing such as bending.
The present inventors have found that a powder coating composition comprising a specific powder which contains specific amounts of a specific vinylidenefluoride copolymer and a specific methyl methacrylate copolymer can provide a coating film being excellent particularly in bending resistance.
Namely, an object of the present invention is to provide a powder coating composition which is superior in dispersibility of pigment, weather resistance, appearance and stain-proofing property and particularly excellent in bending resistance, which makes it possible to give a coating film being hardly cracked during bending process of the metal plate with the film thereon in the application for PCM.
The present invention relates to a powder coating composition comprising a powder which contains
100 parts by weight of a vinylidene fluoride copolymer having a melting point of not higher than 150xc2x0 C., a crystallinity of not higher than 35% and a weight average molecular weight of from 1xc3x97104 to 5xc3x97105 and 10 to 400 parts by weight of a methyl methacrylate copolymer having a glass transition temperature of not higher than 110xc2x0 C. and a weight average molecular weight of from 1xc3x97104 to 5xc3x97105;
an average particle size of particles constituting the powder being from 1 to 100 xcexcm and an apparent density of the powder being from 0.2 to 1 g/ml.
As the specific vinylidene fluoride (VdF) copolymer which can be employed in the present invention, there is, for example, a copolymer obtained by copolymerizing VdF as an essential component with one or more monomers copolymerizable with VdF, for example, a fluoroolefine monomer such as tetrafluoroethylene (TFE), trifluoroethylene (TrFE), chlorotrifluoroethylene (CTFE) or hexafluoropropylene (HFP) or a monomer having an unsaturated bond such as perfluorobutenoic acid, maleic acid or vinyl acetate. By using such a copolymer, there are superior effects, which cannot be obtained by VdF homopolymer, that a coating film can be obtained at a relatively low temperature of not higher than 200xc2x0 C. and occurrence of crack at the time of post processing of a coated metal plate can be prevented.
Among the VdF copolymers, from the viewpoints of good copolymerizability, weather resistance and thermal stability, VdF-TFE copolymer, VdF-TFE-HFP copolymer, VdF-TFE-CTFE copolymer, VdF-TFE-TrFE copolymer, VdF-CTFE copolymer, VdF-HFP copolymer, VdF-TFE-perfluorobutenoic acid copolymer and VdF-TFE-maleic acid copolymer are preferable, and VdF-TFE-HFP copolymer and VdF-TFE-CTFE copolymer are further preferable. Particularly by using a copolymer containing CTFE, balance between hardness of a coating film and post-processability can be improved.
A copolymerization ratio of VdF in the VdF copolymer is, from the viewpoint of good compatibility with methyl methacrylate copolymer, not less than 60% by mole, preferably not less than 70% by mole. From the viewpoint of post processability, the upper limit is preferably 98% by mole.
A melting point of the VdF copolymer is not higher than 150xc2x0 C., preferably 40xc2x0 to 120xc2x0 C. from the viewpoints that leveling property is good and superior appearance, gloss and workability in coating can be obtained even if the powder coating composition obtained from the copolymer is not heated to a high temperature.
A crystallinity of the VdF copolymer is, due to the same reason mentioned above with respect to the melting point, not higher than 35%, preferably from 0 to 10%.
A weight average molecular weight of the VdF copolymer is not less than 1xc3x97104, from the viewpoints that in application such as PCM, crack hardly occurs during bending and post processability is not lowered, and is preferably not more than 5xc3x97105 from the viewpoints that flowability of the powder coating composition is not easy to be lowered and appearance of the coating film is not easy to be damaged.
As the specific methyl methacrylate copolymer which can be employed in the present invention, there are, for example, homopolymer of methyl methacrylate (MMA), a copolymer obtained by copolymerizing MMA as the main component with one or more monomers copolymerizable with MMA such as acrylic acid ester monomer and methacrylic acid ester monomer, and the like.
As the acrylic acid ester monomer, there are, for example, methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate, isobutyl acrylate, t-butyl acrylate, n-hexyl acrylate, cyclohexyl acrylate, benzyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate and the like.
As the methacrylic acid ester monomer, there are, for example, methyl methacrylate, ethyl methacrylate, propyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate (t-BMA), n-hexyl methacrylate, cyclohexyl methacrylate, benzyl methacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate and the like.
As the monomer copolymerizable with the above-mentioned MMA, there are, for example, further to the above-mentioned monomers, conjugated diene compounds such as 1,3-butadiene, isoprene and chloroprene; aromatic vinyl compounds such as styrene, xcex1-methylstyrene, halogenated styrene and divinylbenzene; vinyl cyanide compounds such as acrylonitrile and methacrylonitrile; and the like.
By using such a copolymer, good effects can be obtained in appearance, thermal stability and weather resistance of the coating film.
In the present invention, the purposes are accomplished even if crosslinking is not carried out and, however, the powder coating composition of the present invention can be used in the form of a crosslinkable type by addition of a crosslinkable group and a curing agent in an amount where post processability which is characteristic of the present invention is not damaged.
Further in the present invention, as the monomer copolymerizable with MMA, further to the above-mentioned monomers, for example, a monomer containing one or more reactive groups such as 
can be used.
As the above R, there are, for example, a saturated hydrocarbon residue having 1 to 3 carbon atoms and the like.
Among the above-mentioned reactive groups, xe2x80x94OH, xe2x80x94COOH and 
are preferable.
A MMA copolymer obtained by copolymerizing a monomer having such a reactive group can be used as thermosetting powder coating composition, for example, when used in combination with a curing agent.
As the monomer having such a reactive group, there are, for example, unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid and crotonic acid; amide compounds such as acrylamide, methacrylamide N-methyl acrylamide, N-methyl methacrylamide, N-methylol acrylamide, N-methylol methacrylamide, N-alkyl acrylamide, N-alkyl methacrylamide, N,N-dialkyl acrylamide and N,N-dialkyl methacrylamide; acrylic acid esters such as 2-hydroxyethyl acrylate, N,N-dialkylaminoethyl acrylate and glycidyl acrylate; methacrylic acid esters such as 2-hydroxyethyl methacrylate, N,N-dialkylaminoethyl methacrylate, glycidyl methacrylate (GMA) and ethylene glycol dimethacrylate; vinyl ether compounds such as allylglycidyl ether; hydroxyl-containing monomers such as hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate and hydroxypropyl methacrylate; silicon-containing monomers such as xcex3-trimethoxysilane methacrylate and xcex3-triethoxysilane methacrylate.
The copolymerization ratio of MMA in the MMA copolymer is not less than 70% by mole, preferably 90 to 100% by mole from the viewpoints that compatibility with the VdF copolymer is good and that gloss of the coating film to be obtained is not easy to decrease.
The copolymerization ratio of the monomer in case where the monomer having the reactive group is copolymerized in the MMA copolymer is not more than 20% by mole, preferably 0.1 to 10% by mole from the viewpoint that post processability is good.
The glass transition temperature of the MMA copolymer is not higher than 110xc2x0 C., preferably from 80xc2x0 to 110xc2x0 C. from the viewpoint that workability in coating process is good.
The weight average molecular weight of the MMA copolymer is preferably from 1xc3x97104 to 5xc3x97105 from the viewpoints that post processability is not easy to be worse and that appearance of the coating film is not easy to be damaged.
The above-mentioned VdF copolymer and MMA copolymer can be obtained by a conventional method.
In case of emulsion polymerization or suspension polymerization, there is, for example, a method which comprises removing a polymerization solvent after the polymerization is carried out, washing, drying and pulverizing.
And there is a method comprising drying and granulating with a spray dry after the polymerization.
In case of the emulsion polymerization, there is a method comprising adding an organic solvent to an obtained aqueous dispersion, agitating for granulation and, then, drying the granulated particles.
In case of solution polymerization, there is a method comprising pulverizing after evaporating an obtained solution to dryness or dispersing the obtained solution into a poor solvent of the copolymer, collecting the precipitated copolymer, drying and pulverizing.
Also, in case of bulk polymerization, there is a method comprising pulverizing an obtained bulk copolymer.
The specific powder of the present invention can be obtained by a conventional method, for example, a dry-blend method, from the VdF copolymer and the MMA copolymer prepared through the above-mentioned method.
As other method for obtaining the powder, there is, for example, a method comprising mixing an aqueous dispersion of VdF copolymer with an aqueous dispersion of MMA copolymer, both being obtained by the above-mentioned emulsion polymerization or suspension polymerization, co-coagulating and drying to give a desired powder or, further, pulverizing the dried resin.
As the mixing ratio of the VdF copolymer and the MMA copolymer, to 100 parts (parts by weight, hereinafter the same) of VdF copolymer, 10 to 400 parts, preferably 20 to 250 parts of MMA copolymer is admixed.
By adopting such a mixing ratio, adhesion of the powder coating composition to a substrate, bending resistance, gloss, weather resistance, water-proofing property and chemical resistance of the coating film obtained from the composition are hardly injured.
Further, as other method for obtaining the powder, there is, for example, a method wherein a monomer mixture comprising MMA and a monomer copolymerizable with MMA is seed-polymerized in an aqueous dispersion containing particles (hereinafter may be referred to as xe2x80x9cseed particlesxe2x80x9d) of VdF copolymer and an aqueous dispersion obtained by the seed polymerization is subjected to, for example, spray-drying or coagulation, drying and, if necessary, pulverizing to give the powder.
The VdF copolymer used as the seed particle can be obtained by a conventional emulsion polymerization as mentioned above. The powder can be obtained by emulsion-polymerizing, in an aqueous dispersion containing the seed particles, a monomer mixture comprising VdF and a monomer copolymerizable with VdF in the coexistence of not more than 1.0% by weight, preferably not more than 0.5% by weight, more preferably not more than 0.2% by weight (the lower limit is usually 0.01% by weight) of a fluorine-containing surfactant on the basis of water and 0.001 to 0.1% by weight, preferably 0.01 to 0.05% by weight of a non-fluorine-containing nonionic surfactant on the basis of water. This dispersion can contain the seed particles having the above-mentioned average particle size in a high concentration of 30 to 50% by weight. An amount of the fluorine-containing surfactant of more than 1.0% by weight is not preferable since there occurs phenomenon such as precipitation of the surfactant on a coating film when the coating film is made from the dispersion, and water absorption of the coating film tends to be increased and water proofing property tends to lower. An amount of the non-fluorine-containing nonionic surfactant of more than 0.1% by weight is not practical since, polymerization rate is lowered through chain transfer and reaction is stopped. In case of less than 0.001% by weight, there is almost no effect of reducing an average particle size of particles after polymerization. The polymerization temperature is 20xc2x0 to 120xc2x0 C., preferably 30xc2x0 to 70xc2x0 C. When the polymerization temperature is lower than 20xc2x0 C., stability of the produced latex is generally lowered and when the polymerization temperature is higher than 120xc2x0 C., decrease in polymerization rate through chain transfer tends to occur. The polymerization is, depending on the kind of polymer, usually carried out by heating under a pressure of 1.0 to 50 kgf/cm2 (gauge pressure) for 5 to 100 hours.
As the monomer copolymerizable with VdF used here, there are the above-mentioned monomers and the same copolymerization ratio can be employed.
As the fluorine-containing surfactant used for emulsion polymerization of the seed particles, there is one or a mixture of compounds containing fluorine atom in its structure and having surface activity. There are, for example, acid represented by X(CF2)nCOOH (n is an integer of 6 to 20, X is F or H) and an alkali metal salt, ammonium salt, amine salt or quaternary ammonium salt thereof; acid represented by Y(CH2CF2)mCOOH (m is an integer of 6 to 13, Y is F or Cl) and an alkali metal salt, ammonium salt, amine salt or quaternary ammonium salt thereof. More in concrete, there are ammonium salt of perfluorooctanoic acid, ammonium salt of perfluorononanoic acid and the like. Further, conventional fluorine-containing surfactants can be also used.
As the non-fluorine-containing nonionic surfactant used for emulsion polymerization of the seed particles, there are, for example, polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl esters, sorbitan alkyl esters, polyoxyethylene sorbitan alkyl esters, glycerol esters and derivatives thereof. More in concrete, examples of the polyoxyethylene alkyl ethers are, for instance, polyoxyethylene lauryl ether, polyoxyethylene cetyl ether, polyoxyethylene stearyl ether, polyoxyethylene oleyl ether, polyoxyethylene behenyl ether and the like. Examples of the polyoxyethylene alkyl phenyl ethers are for instance, polyoxyethylene nonyl phenyl ether, polyoxyethylene octyl phenyl ether and the like. Examples of the polyoxyethylene alkyl esters are, for instance, polyethylene glycol monolaurate, polyethylene glycol monooleate, polyethylene glycol monostearate and the like. Examples of the sorbitan alkyl esters are, for instance, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monooleate and the like. Examples of the polyoxyethylene sorbitan alkyl esters are, for instance, polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monopalmitate, polyoxyethylene sorbitan monostearate and the like. Examples of the glycerol ethers are, for instance, glyceryl monomyristate, glyceryl monostearate, glyceryl monooleate and the like. Examples of the derivatives of the above-mentioned compounds are, for instance, a polyoxyethylene alkyl amine, a polyoxyethylene alkyl phenyl-formaldehyde condensate, a polyoxyethylene alkyl ether phosphate and the like. Particularly preferable are polyoxyethylene alkyl ethers and polyoxyethylene alkyl esters which have HLB value of 10 to 18. Examples thereof are polyoxyethylene lauryl ether (EO: 5 to 20, EO represents the number of ethylene oxide units), polyethylene glycol monostearate (EO: 10 to 55) and polyethylene glycol monooleate (EO: 6 to 10).
In the presence of the so-obtained seed particles, a monomer mixture containing MMA and a monomer copolymerizable with MMA is seed-polymerized.
As the monomer copolymerizable with MMA used here, there are, for example, the above-mentioned monomers.
The amount of MMA on the basis of the mixture of MMA and the copolymerizable monomer is not less than 70% by mole, preferably 90 to 100% by mole. By using MMA within the range, compatibility with the seed particles is better and gloss of the obtained coating film is not easy to be lowered.
The total amount of MMA and the monomer copolymerization with MMA is 10 to 400 parts, preferably 20 to 250 parts per 100 parts of the seed particles. By using MMA and the monomer within the range, adhesive property of the powder coating composition to substrate and bending resistance, gloss, weather resistance, water proofing property and chemical resistance of the coating film obtained from the composition are not easy to be lowered.
The seed polymerization can be carried out under the same conditions as in a conventional emulsion polymerization. For example, into an aqueous dispersion containing the seed particles are added a surfactant, polymerization initiator, chain transfer agent, and if necessary, chelating agent, pH modifier and solvent, and reaction is carried out at 20xc2x0 to 90xc2x0 C., preferably 20xc2x0 to 80xc2x0 C., more preferably 30xc2x0 to 70xc2x0 C. for 0.5 to 6 hours.
In the seed polymerization, each of a method wherein a whole amount of monomers are introduced at once into a reaction system, a method wherein a certain portion of monomers is introduced and reacted and then the remain is introduced continuously or dividedly, and a method wherein a whole amount of monomers is introduced continuously can be employed.
It is assumed that when a mixture of MMA and the monomer copolymerizable with MMA is seed-polymerized in the presence of the seed particles through emulsion polymerization method, the swelling of the seed particle with these monomers occurs, which results in an aqueous dispersion in which the seed particle is dissolved homogeneously in these monomers, and then these monomers are polymerized by the addition of an polymerization initiator to form a particle wherein molecular chains of MMA copolymer and molecular chains of VdF copolymer are compatibly entangled with each other. It is also assumed that pseudo-interpenetrating polymer network (S-IPM) is formed by copolymerizing a multifunctional monomer. As the multifunctional monomer, there are, for example, monoglycol dimethacrylate and diglycol dimethacrylate. With these monomers, a powder coating composition having better performances can be obtained.
As the surfactant in the present invention, an anionic surfactant, a nonionic surfactant, or a mixture thereof can be used and an amphoteric surfactant can also be used. As the anionic surfactant, a sulfate of higher alkyl alcohol such as sodium salt of an alkyl sulfonate, an alkylbenzene sulfonate, a dialkyl succinic acid ester sulfonate, an alkyldiphenyl ether disulfonate or the like can be used. As the nonionic surfactant, the above-mentioned non-fluorine-containing nonionic surfactant can also be used. As the amphoteric surfactant, lauryl betaine or the like can be used. Also, there can be used so-called reactive emulsifier, which is copolymerizable with MMA and the monomer copolymerizable with MMA, such as sodium styrene sulfonate or sodium alkylsulfosuccinate. An amount of the surfactant or reactive emulsifier is usually about 0.05 to 5.0 parts per 100 parts of the total of MMA and the monomer copolymerizable with MMA.
The polymerization initiator is not particularly limited if it generates radicals at 20xc2x0 to 90xc2x0 C. which can be subjected to free radical reaction in aqueous medium. As the case may be, it is possible to use the initiator in combination with a reducing agent. Usually, examples of the water-soluble polymerization initiator are a persulfate and hydrogen peroxide and examples of the reducing agent are sodium pyrobisulfite, sodium hydrogensulfite and sodium L-ascorbate. Examples of the oil-soluble polymerization initiator are, for instance, diisopropyl peroxydicarbonate (IPP), benzoyl peroxide, dibutyl peroxide and azobisisobutyronitrile (AIBN). An amount of the polymerization initiator is usually about 0.05 to 2.0 parts per 100 parts of the total of MMA and the monomer copolymerizable with MMA.
A polymerization temperature is 20xc2x0 to 90xc2x0 C., preferably 30xc2x0 to 70xc2x0 C.
As the chain transfer agent, a halogenated hydrocarbon (for example, chloroform, carbon tetrachloride), a mercaptan (for example, n-dodecylmercaptan, t-dodecylmercaptan, n-octylmercaptan or the like can be used. An amount of the chain transfer agent is usually about 0 to 5.0 parts per 100 parts of the total of MMA and the monomer copolymerizable with MMA.
As the solvent, methyl ethyl ketone, acetone, trichlorotrifluoroethane, methyl isobutyl ketone, ethyl acetate or the like can be used in such a small amount that workability, safety for accident, safety for environment and safety in production process becomes not worse. By the addition of the solvent, swelling property of the seed particle by the monomer may be improved.
As described above, the particle constituting the powder can be obtained by, for example, the dry blend method, the method of mixing two kinds of aqueous dispersions and coagulating, the method of seed polymerization, the method of spray-drying the aqueous dispersion of the seed polymer or, coagulating and drying. An average particle size of the particle is 1 to 100 xcexcm, further 1 to 10 xcexcm. When the average particle size is less than 1 xcexcm, it is difficult to control thickness of the coating film due to electrostatic repulsion, and foaming easily occurs to cause the lowering of appearance. When more than 100 xcexcm, leveling property and appearance of the coating film are lowered.
The powder has an apparent density of 0.2 to 1 g/ml. When the apparent density is within the range, appearance of the coating film and coating workability are good. If the apparent density is less than 0.2 g/ml, leveling property becomes worse to make the coating film uneven. Also, if more than 1 g/ml, adhesion of the coating powder becomes so worse that workability of thick coating is lowered and it becomes difficult to handle the powder.
Since the powder has a softening temperature of not less than 40xc2x0 C., each particle does not stick to each other when stored at ordinary temperature and stability of the powder is good. It should be noted that, in the present specification, the softening temperature means a glass transition point or melting point of not lower than 40xc2x0 C. which the obtained resin composition has.
In the present invention, the MMA copolymer obtained by copolymerizing the monomer having a reactive group as mentioned above can be used in combination with a curing agent.
As the curing agent, for example, block isocyanate such as xcex5-caprolactam isocyanate, isophoronediisocyanate, tolylenediisocyanate, xylylenediisocyanate, 4,4-diphenylmethanediisocyanate or hexamethylenediisocyanate is preferable and a curing agent which is in solid state at ordinary temperature is particularly preferable. An amount of the curing agent is preferably from 0.1 to 5 parts per 100 parts of the MMA copolymer or the monomer having a reactive group.
As the powder coating composition, the powder comprising the VdF copolymer and the MMA copolymer may be used as it is, or the powder can be used after mixing with the curing agent and additives which are employed for usual powder coating compositions.
As the additives for powder coating, there are, for example, pigment, surface modifier, organic solvent, plasticizer and the like.
The powder coating composition of the present invention can be applied to various kinds of substrates. The coating method is, for example, a usual electrostatic coating method wherein the composition is applied by airless spray on a metal plate to which specific voltage is applied, and then sintered.
Examples of the substrate or article are, for instance, exterior goods for architecture such as panel for external wall, sash, roof material and fence, exterior goods for civil construction such as chemical plant, pipe, erection frame, guard rail and steel tower, exterior goods of car, ship, switchboard, outdoor machine of air conditioner and electric appliance, and exterior goods for antenna and agricultural machinery.
As the powder coating composition of the present invention, the following combination is preferable.
This composition is advantageous from the viewpoint that the coating film superior in post processing and stain-proofing property can be formed by heating to not higher than 200xc2x0 C.
Further preferable combination is as follows.
This composition is superior in appearance of the coating film in addition to the above-mentioned characteristics.